Painting dirt for fun and no profit...
A couple of fellows wrote to me offline about this latest bit of craziness-painting on dirt. Well, first off, it ain't dirt like what your flowers grow in or what's on the floor after grandkids drop by. What I use is actually "crushed stone"-rocks that have been broken up, run over by heavy trucks, etc., and washed down the alley into the street and along the curbs.
Here in Texas, we have a white/gray "limestone" called Austin Chalk and it is found under "regular" dirt, in creek and river beds, and is used to pave gravel roads and even graded out before concrete foundations are poured. This is what I use. It is nice and light in color and can be "stained" for any variations. Behind my house, the trash trucks prowl the alley and crush the stone down into small and very small stones and very fine powder...just what I want to represent the dusty area I model in New Mexico. I also have some very nice reddish/pink stone from Silver City, NM that was sent to me by Duane Ericson. I sometimes mix these together, even dirt isn't the same color all the time.
So, I have some large areas to apply dirt & rocks and got tired of gluing small areas of dirt with a spoon and some 50/50 Elmer's & water. I want to get scenery done before St. Peter calls. The following photos may best describe what I devised to quicken the dirtification process.

The photos explain it better than I can. Try this and you may throw away your eye dropper and tweezers.

Duane-I found that the "slurry" needs to be about as thinned as latex house paint. Liberal spray-down of "wet" water makes it run down gullies, etc. and looks quite realistic-it does like real dirt does (well, it IS real dirt) and makes rivulets and gets into cracks like it should.
Try it...you'll like it (copyrighted commercial line)

It likely will depend on the particular dirt. What we have around here, riverbottom black dirt, and a slightly clay-ey dirt almost the same color as Model Master "Earth" color paint will probably react differently.

____________________See y'all later, Forrest.Screw the rivets, I'm building for atmosphere

Maybe what I call "dirt" ain't your old out in the back yard dirt. What I am using is really stone, crushed into powder. You couldn't grow plants in this, it isn't soil but rock. I look at areas where there are gravel roads and driveways and get the "raw material" from there. The almost black dirt that is in my yard isn't suitable for what I want...too dark and it does become mud when wet. The stone resembles mud when wet but when dry it looks totally different. It pays to look for the un-obvious while walking around the neighborhood. The biggest problem I have with "material collection" is the neighbors looking at me sideways while I scrape up supplies from the street and alley. But then they know that I am crazee and build model trains.